ics212-20-fileprocessing

Transcript

1. ICS 212 Program Structure William McDaniel Albritton M.S.
2. Memory Allocation  File Basics  Sequential Access Files  Text Files and Binary Files  Random-Access Files
4. File Basics
5. Data Hierarchy  Byte: 8 bits (ASCII character ‘A’ = 01000001)  Field: Group of characters (character string “Fred”)  Record: Composed of related fields (struct)  File: Group of related records (student record file)  Database: Group of related files (students, faculty, and staff files)
6. What is a Record?  Record is a group of related fields  For example, in the C language, a structure is the way to define a record  Special field called a “record key” or “primary key” is used to help retrieve a specific record
7. What is a File?  File is a group of related records  A file stores records ordered by the record key field or primary key field  The C language views a file as a sequential stream of bytes  Files end with an end-of-file (EOF) marker
8. End-of-File Marker  File ends with a end-of-file (EOF) marker  EOF key combinations on different systems:  UNIX and Mac: Ctrl-D; PC: Ctrl-Z  Often used to loop through a file (see write.c) while(EOF != character){ fputc(character, filePointer); character = fgetc(stdin); }
9. Streams  Three file streams (file pointers) are automatically provided in the C language  Standard input stream to keyboard (stdin)  Standard output stream to screen (stdout)  Standard error stream to screen (stderr)
10. Read a Character from a File  int fgetc(FILE *filePointer)  Reads one character at a time from a file  Similar to function getchar()  On success, returns character  If EOF or error, returns EOF char character1 = fgetc(stdin); char character2 = getchar();
11. Write a Character to a File  int fputc(int character, FILE *filePointer);  Writes one character at a time to a file  If successful, returns the character  If not successful, returns EOF  Similar to function putchar() error = fputc(character1, stdout); error = putchar(character1);
12. Write Formatted Output to a File  int fprintf(FILE *filePointer, const char *format, arg1, arg2, arg3, …);  Writes formatted output to a file  Similar to function printf() fprintf(stdout,“%c”,character1); printf(“%c”,character1);
13. Example Program  See example code at: streams.c  Can redirect the input (<) and output (>) to files  Can use append (>>) to output % cp streams.c program.c % make –f makefile-program % ./program <char.txt >out.txt % more out.txt
15. Sequential Access Files
16. Sequential Access Files  Individual records are not fixed in length  Have to do a search through the entire file to find something
17. File Structure  FILE is a structure defined in <stdio.h>  Contains data used to process a file  Location of a buffer  Current character position in the buffer  Whether file is being read or written  Whether errors or EOF have occurred
18. Function fopen() to Open a File  FILE *fopen(char *fileName, char *fileMode);  1st argument: name of the file  2nd argument: how to use the file (read, write, etc.)  Function fopen() returns a pointer to a FILE structure  If error (such as no such file exists) will return NULL FILE *filePointer = NULL; filePointer = fopen(“file.txt”,“w”);
19. File Modes  Mode "r"  Open a file for reading  Mode "w"  Open or create a file for writing  If file already exists, erase the contents
20. File Modes  Mode "a"  Append: open or create a file  Write to the end of the file  Mode "r+"  Open a file for updating (reading and writing)
21. File Modes  Mode "w+"  Open or create a file for updating  Erase contents of existing file  Mode "a+"  Append: open or create a file for updating  All writing done at end of file
22. Function fclose() to Close a File  int fclose(FILE *filePointer);  Closes a file  Returns 0, if successful  Returns EOF, if not successful  Will be done automatically after a program finishes int fileClosed = fclose(filePointer);
23. Example Program  See example code at: write.c  Reads text from the keyboard and writes the text to a file called “file.txt” % cp write.c program.c % make –f makefile-program % ./program (type some text) % more file.txt
24. Function fputs() Writes String to File  int fputs(const char *string, FILE *filePointer);  Writes a string to a file  Returns a nonnegative integer for success  Returns EOF for error int error = 0; error = fputs(“abc”, filePointer);
25. Function fgets() Reads String from File  char *fgets(char *string, int max, FILE *filePointer);  Reads a line from a file, until reads max-1 characters, newline, or EOF  Stores all the characters (including '\n') from the file into the string, which ends with NULL character ('\0')  If error, returns NULL; if success, returns pointer to string
26. Buffer Overflow Attack  Do NOT use: char* gets(char *string);  Reads one line from standard input, and puts it into array “string”, including blank spaces  Should not be used because of the potential for “buffer overflow” attack  Entering too many characters can crash the program, write over memory, and/or run code on your computer
27. Buffer Overflow Attack Prevention  char *fgets(char *string, int max, FILE *filePointer);  Use the fgets() function instead, which limits the number of characters that can be entered  See example code at: inputline.c char string[MAX] = {‘\0’}; fgets(string, MAX, stdin); printf(“%s”, string);
28. Function feof() Detects End of File  int feof(FILE *filePointer);  Returns non-zero, if end-of-file (EOF)  Returns 0, if not end-of-file (EOF) while(0 == feof(filePointer)){ //file processing code . . . }
29. Function rewind() Used to Reread a File  void rewind (FILE *filePointer);  Once a file is read, the file position pointer is at the end of the file, so this resets it to the beginning of the file  File position pointer is an integer value, which is the byte location in the file for next read or write rewind(filePointer);
30. Example Program  See example code at: read.c  Reads from a file called “file.txt” and displays contents of file on the screen two times % cp read.c program.c % make –f makefile-program % ./program
32. Text Files and Binary Files
33. Text Files  Store ASCII characters  Such as character ‘A’ = 01000001  ASCII uses only 7 bits, 8th bit is for error control  Easy to read, but take up more space  Example text files: file.txt, webpage.html, program.c
34. Binary Files  Store character and non-character data  Compact, but more complex to read  Example binary files: photo.gif, program.exe, paper.docx, slides.pptx
35. Binary Files  Add a ‘b’ for binary file modes  rb, wb, ab, rb+, wb+, rb+  Possible errors  Reading/transferring a binary file as a text file may corrupt the 8th bit
37. Random-Access Files
38. What are Random-Access Files?  Files that are organized into records, which are fixed in length  Can be access a specific record without searching though other records  Much quicker to access data than sequential access files
39. How Do Random-Access Files Work?  Use a record key to locate a record by calculating the distance from the beginning of the file  Can update and delete data in the file without overwriting other data in the file
40. Function fwrite()  size_t fwrite(const void *buffer, size_t size, size_t count, FILE *filePointer);  buffer = address of variable or 1st element in array  size = size of each object in bytes  count = number of objects to be written  filePointer = the file to send the data to  Returns the number of objects written successfully
41. Creating Random-Access Files  Use function fwrite() to create the file and to set up the file, so records can be easily added later on  See example program: creating.c  Creates 20 student records in a file called “students.txt” % cp creating.c program.c % make –f makefile-program % ./program
42. Function fseek()  int fseek(FILE *stream, long int offset, int whence)  stream = file pointer to be set to a given offset  offset = number of bytes from “whence”, which is:  SEEK_SET (from beginning of file)  SEEK_CUR (from current location)  SEEK_END (from end of file)  If success, return zero; if error, return non-zero
43. Writing to Random-Access Files  Use fseek() to position the file position pointer when writing to a file  See example program: writing.c  Writes student records to a file % make –f makefile-writing % ./program
44. Function fread()  size_t fread(void *buffer, size_t size, size_t count, FILE *filePointer);  buffer = address of array or variable that stores objects  size = size of each object in bytes  count = number of objects to be read  filePointer = the file from which the memory is read  Returns number of objects read successfully
45. Reading from a Random-Access Files  Use fread() to read a specific number of bytes from a file into memory  See example program: reading.c % cp reading.c program.c % make –f makefile-program % ./program
46. Memory Management  File Basics  Sequential Access Files  Text Files and Binary Files  Random-Access Files